Known wave gear devices include cup-type wave gear devices provided with a cup-shaped flexible externally toothed gear, and silk hat-type wave gear devices provided with a silk-hat-shaped flexible externally toothed gear. In a cup-shaped flexible externally toothed gear, external teeth are formed on the outer peripheral surface part at the portion, on the side of a front opening end, of a cylindrical barrel part which is capable of deflecting to a non-circular shape; a diaphragm extending radially inwards is formed on a rear opening end; and a disc-shaped or ring-shaped boss is formed on the inner peripheral edge of the diaphragm. The diaphragm and the boss correspond to the bottom portion of the cup shape. The flexible externally toothed gear is fastened, through the boss, to a member such as a load-side member. In a silk-hat-shaped flexible externally toothed gear, external teeth are formed on the outer peripheral surface portion, on the front opening end, of a cylindrical barrel part; a diaphragm extending radially outwards is formed on the rear opening end; and a ring-shaped boss is formed on the outer peripheral edge of the diaphragm. The diaphragm and the boss correspond to the flange portion of the silk hat shape.
In a cup-type wave gear device or a silk hat-type wave gear device, contact parts to be lubricated are lubricated by a lubricant packed into the interior space in the flexible externally toothed gear. Patent Document 1 proposes a method for disposing an oil tank inside the flexible externally toothed gear. Small holes are formed on, e.g., on the outer peripheral surface of the oil tank. When the oil tank rotates with the flexible externally toothed gear, oil stored in the tank is discharged from the holes into the interior space in the flexible externally toothed gear. The lubricant discharged into the interior space lubricates various sections.